BY DR EMILY BALDWIN
Posted: 19 December, 2008
Scientists working with Mars Reconnaissance Orbiter (MRO) data have spotted a long sought after mineral on the Martian surface and, with it, unexpected clues to the Red Planet's watery past.
The mineralogical treasure comes in the form of carbonates, rocks that are created when water and carbon dioxide interact with calcium, iron or magnesium in volcanic rocks. On Earth, carbonate rocks such as limestone and chalk dissolve quickly in acid, so their survival on Mars to the present day challenges suggestions that an exclusively acidic environment once dominated the planet. Furthermore, it provides strong evidence that different types of watery environments existed over time and in different regions across the Red Planet, enhancing the chances that one or more of these locations could have supported the goldilocks 'just-right' conditions for life.
Carbonate, which is indicative of a wet and non-acidic history, occurs in very small patches of exposed rock appearing green in this colour representation of an area about 20 kilometers wide on Mars. Image credit: NASA/JPL/JHUAPL/MSSS/Brown University.
"We're excited to have finally found carbonate minerals because they provide more detail about conditions during specific periods of Mars' history," says Scott Murchie, principal investigator for the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), the MRO instrument that detected the carbonates in layers of rock.
The results suggest that Mars had neutral to alkaline water when the minerals formed at these locations more than 3.6 billion years ago. "This discovery of carbonates in an intact rock layer, in contact with clays, is an example of how joint observations by CRISM and the telescopic cameras on the Mars Reconnaissance Orbiter are revealing details of distinct environments on Mars," says Sue Smrekar, deputy project scientist for the orbiter.
The researchers report clearly defined exposures of the carbonate mineral in layers of rock surrounding the 1,489 kilometre diameter Isidis impact basin, which formed more than 3.6 billion years ago. The clearest examples are exposed at the edge of the basin, along a 666 kilometer long trough system called Nili Fossae. This region is enriched in olivine, a mineral that can react with water to form carbonate.
The presence of carbonate rocks also has implications for the evolution of a planet’s atmosphere, since carbon dioxide is locked up inside these rocks. If all of the carbon dioxide locked in Earth's carbonates were released, our atmosphere would be thicker than even that of Venus. Some researchers believe that a thick, carbon dioxide rich atmosphere kept ancient Mars warm and sustained liquid water on its surface long enough to have carved the valley systems observed today.
Instruments onboard the Mars Reconnaissance Orbiter have pinpointed the locations of carbonate outcrops on Mars. Image: NASA/JPL.
"The carbonates that CRISM has observed are regional rather than global in nature, and therefore, are too limited to account for enough carbon dioxide to form a thick atmosphere," says Bethany Ehlmann, lead author of the article that appears in today’s issue of the journal Science. "Although we have not found the types of carbonate deposits which might have trapped an ancient atmosphere, we have found evidence that not all of Mars experienced an intense, acidic weathering environment 3.5 billion years ago, as has been proposed. We've found at least one region that was potentially more hospitable to life."
This is not the first time that carbonates have been spied on Mars: NASA's Phoenix Mars Lander tasted them in soil samples earlier this year and researchers had previously found them in Martian meteorites that fell to Earth, and in windblown Mars dust observed from orbit. However, the dust and soil could be mixtures derived from many areas, so their origins have been unclear. The latest MRO observations indicate carbonates may have formed over extended periods on early Mars and for the first time point to specific locations where future rovers and landers could search for possible evidence of past life.
MRO has now completed its primary two-year science phase and begins another two-year phase imminently, corresponding to another Martian year.